Prickly porcupine quills may hold clues for medical technology

Nature has a history of inspiring man-made materials: look to Velcro and swimsuits.

If you’re unlucky enough to have met the business end of a porcupine—or if you have a pesky dog that has—you probably know a thing or two about porcupine quills. Scientific research is now catching up: a new study in PNAS takes a closer look at porcupine quills. It illustrates why they penetrate skin and muscle so easily, and why they are so difficult to remove. Furthermore, it suggests the structure of these quills may have useful medical applications.

North American porcupines have as many as 30,000 quills that are ready to be deployed when a predator comes into contact with the porcupine (contrary to popular belief, the quills can't be shot through the air). Unlike the quills of hedgehogs, echidna, and even the African porcupine, the tips of the quills of North American porcupines are covered in overlapping backward-facing barbs.

To figure out how these barbs affected quill function, the researchers compared normal North American porcupine quills to quills that had been carefully sanded until they were smooth and barb-free. The smooth quills required more than fifty percent additional force to penetrate tissue than barbed quills did, and they actually caused more damage as they entered tissue. Barbs help quills enter cleanly, since they create high stress concentration points in the tissue, lessening the overall force needed for penetration. Serrated knives operate under a similar principle, helping them cut cleanly and easily through food (or skin, if you’re a surgeon or serial killer).

Once a North American porcupine quill is embedded in tissue, the barbs make it difficult to remove. Quills with barbs required 0.33 N more force to remove than those without barbs. It turns out the tiny barbs are either deployed or bent once the quill is in the tissue, interlocking with the tissue fibers and increasing resistance. The researchers found that the barbs within 1mm of the tip of the quill make the most difference in the “pull-out force” needed, although barbs all along the first 4mm of the quill cooperate in increasing the difficulty of extraction.

The researchers realized they could apply these findings to medical technology to increase the efficacy of various medical devices. Natural innovations have a long history of inspiring man-made materials. Velcro was inspired by the sticky burrs of the burdock plant, and high-performance swimsuits are based on the hydrodynamic qualities of shark skin. Similarly, porcupine quills could serve as models for materials that need to either enter the skin cleanly and easily, or stay adhered to the skin.

The researchers actually built two medical prototypes to illustrate the efficacy of the porcupine quills. First, they created a polyurethane hypodermic needle with microscopic barbs; this needle required 80 percent less force to penetrate muscle tissue than a similarly shaped needle without barbs. Then they created a patch with microscopic quills on its underside, which was able to adhere incredibly tightly to pig skin by interlocking with the tissue. It took thirty times more work to remove the barbed patch than a similar patch without barbs. Patches like these could be extremely effective in delivering anesthesia or other medication, or in joining tissue together.

The study, however, doesn't address the amount of tissue damage that occurs when the quills—or the man-made barbed patch—are pulled out of skin or muscle.

If the benefit of the barbs in penetrating tissue is creating high-pressure points similar to serrations on a knife, maybe the problem of removal can be solved by making them triangular flanges instead of barbs -- basically a bi-directional "barb" that gives the same benefit to removal as it does to insertion.

I'm not sure about serrated knives causing less damage than a single blade. If the blade is sufficiently sharp there is already sufficient stress concentration to cut without damage. Surgeon's scalpels are, for the most part, straight edges and very, very, very sharp. So sharp that the natural tension of skin tends to pull the flesh away from the cut in almost an explosive manner.

Serrated edges are great when you're cutting into something that yields as you try to cut or something that's not taut - like bread. The compression of the substrate will pack onto one of the tips of the serrated edge where the stress concentration can do its work.

I suspect the reason the quills work better is that they're entering normally to the flesh (nominally) so it's the squishy-substrate problem. The flesh wants to move away from the compression.

That one sailed right over the top of my head until I saw the link. Hilarious.

"The male porcupine rears up on his hind legs, walks toward the female with a fully erect penis, and proceeds to soak her in urine with a spray forceful enough to shoot 6 feet....Scientists call it the Whitten Effect. The Internet calls it a golden shower."

The injury inflicted to the female genitalia upon extraction is actually triggering the ovulation. It also has the corollary advantage to render the "victim" totally uncooperative for any further copulation, a non-negligible competitive advantage for the first mating male.

Serrated knives operate under a similar principle, helping them cut cleanly and easily through food (or skin, if you’re a surgeon or serial killer).

As a point, serrated knives cut most foods less cleanly than a plain edge, all else equal. A serrated knife works as a saw, leaving a relatively ragged cut compared to a well-maintained plain-edged knife. There are, of course, exceptions; bread knives, for instance.

A close analogy would be the pits in e.g. santoku; they reduce the resistance of the blade by allowing air to get between the matter being cut and the side of the blade.

Regarding the use for patches and needles, I shouldn't wonder if there's a biodegradable material that could be used. The needle is pushed through the skin and is left to break down. In the case of patches, I could see it for hard-use bandages, if a material can be made that would break down in a fairly predictable time, like 20 hours +/-2 hours. Would be great when you're sweating to have a non-adhesive bandage, even if you have to wait for it to "rot" off.

Kate Shaw Yoshida / Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas.